1. Field of the Invention
The invention is generally related to electrical bias voltage generation and more specifically to the optical generation of an adjustable, stable, low-noise, electronically isolated bias for use with precision analytical equipment.
2. Description of the Related Art
The generation of bias voltages is widely known in the field of analytical chemistry. Equipment used to detect very small levels of charge use a bias voltage to produce an accelerating field in ion detectors, such as chromatographic ionization detectors.
A chromatographic ionization detector operates by applying a high voltage across discharge electrodes that are located in a gas-filled source chamber. In the presence of a detector gas such as helium, a characteristic discharge emission of photons occurs. The photons irradiate an ionization chamber receiving a sample gas that contains an analyte of interest. Ions are produced in the ionization chamber as a result of photon interaction with ionizable molecules in the sample gas. Such detectors are well known in the art and include U.S. Pat. No. 5,767,683 issued Jun. 16, 1998 to Stearns, Cai and Wentworth, U.S. Pat. No. 5,594,346 issued Jan. 14, 1997 to Stearns and Wentworth, and U.S. Pat. No. 5,541,519 issued Jul. 30, 1996 to Stearns and Wentworth.
The sensitivity and resolution of detection equipment may be limited by the stability of the bias voltage and the extraneous electrical variations, or noise, created by associated electrical circuits. Voltage variations in the bias and/or leakage currents produced by the bias may mask the desired occurrences to be measured.
Simple bias voltage may be generated from a 12 V DC power supply. Transistors and integrated circuit converters are used to modify the frequency and voltage of the current from the power supply to obtain a desired bias. Further transistorized circuitry may be used to filter and monitor the current and voltage in order to achieve a useable degree of stability.
Bias generation in the prior art has typically involved the use of transformer-coupled circuits in which a first transformer, driven by an alternating-current source, is connected to a second transformer whose isolated output is then rectified, filtered, and regulated at a predetermined voltage by additional circuitry. Disadvantage of this scheme include: the output bias voltage is not adjustable without additional feedback circuitry; variations in the output bias voltage are not sensed and regulated without additional feedback circuitry; AC electromagnetic fields may be coupled to the detecting circuitry, causing instability in the measurement process without additional shielding; and the number of components required may increase the cost and reduce the reliability of the employing device.
Diodes are known to be able to produce light when a current is passed through, or to generate a current when excited by a light source. In both cases, the intensity of the light is proportional to the magnitude of the current.
Incident with a current flow through a diode is a voltage drop across the diode. The relationship between the current and the voltage is given by the well-known diode equation:ID=IS eK(T−T0) [eV/λVt−1]where                IS is the saturation current, fixed by the materials and fabrication of the diode (amps);        K is a constant for the material used for the diode, approximately 0.045 for silicon;        T is the diode temperature (°K);        T0 is the diode reference temperature (°K);        Vt is the threshold voltage, 0.026 volts (V);        V is the voltage through the diode (V);        e is the electron charge (1.602×10−19C);        K is Boltzmann's Constant (1.380×10−23 J/K); and        λ is a constant for the material used for the diode, approximately 2 for silicon.Of importance is that diode current and voltage drop are not linearly proportional and are influenced by temperature. For illustration of the influence of temperature, where IS=1.0E−9 amperes, T−T0=0 and V=0.036 volts, then ID=1.0E−9 amperes; in the same example where V=0.36 volts, then ID=1.0E−6 amperes. If at V=0.36 Volts, diode temperature, T, rises such that T−T0=10° C., then ID=1.57E−6 amperes.        
In a practical photovoltaic diode circuit, some type of device or load will be externally connected to the photovoltaic diode. When the effect of such a load is added to the diode equation the equation becomes:ID=ISeK(T−To)[eV/λVt−1]+V/RLwhere ID is the total generated current and RL is the value of the load, in ohms.
Anomalies in a power supply and environmental conditions, such as temperature and humidity affect the electrical current produced by an electrical circuit. The voltage supplied to the load is subject to such anomalies. A practical photovoltaic diode circuit requires some means of control and stabilization of the generated voltage. Some examples of prior art circuits designed to compensate for voltage variations in circuits include:
U.S. Pat. No. 4,375,596, issued on Mar. 1, 1983, to Hoshi, discloses a reference voltage generator circuit, which overcomes variations in a power supply by dividing the power supply voltage to create two output signals, uniformly modifying the signals in opposite polarity, then averaging the resulting signals to generate a constant value of reference voltage.
U.S. Pat. No. 4,380,706, issued on Apr. 19, 1983, to Wrathall, discloses a temperature stable voltage reference source, which uses a differential amplifier with an output coupled to an additional amplifying stage, involving two bipolar transistors, wherein the emitter of one transistor is larger than the emitter of the other transistor. Cascaded emitter followers are used between the two amplifying stages to develop a higher voltage, which is fed back into the inputs of the differential amplifier, thereby establishing a more independently stable reference voltage circuit.
U.S. Pat. No. 4,471,290, issued on Sep. 11, 1984, to Yamaguchi, discloses a substrate bias generating circuit responsive to the output signal of the oscillator circuit, which includes a voltage divider connected between the output terminal of the bias generating circuit and a ground terminal, and a level sensor for producing a control signal to the oscillator circuit when it is detected that the output voltage of the voltage divider reaches a predetermined value, to thereby stop the oscillating operation of the oscillator circuit.
U.S. Pat. No. 5,262,989, issued on Nov. 16, 1993, to Lee et al., discloses a circuit for sensing back-bias levels in a semiconductor device that causes the voltage pump circuit to adjust output to reach and maintain a desired voltage level.
U.S. Pat. No. 3,975,649, issued on Aug. 17, 1976, to Kawagoe et al., discloses a temperature compensation circuit that uses a high value resistor and at least one field-effect transistor for connection between a circuit to be compensated and the power source, such that the when ambient temperature of the circuit increases the current flowing through the field-effect transistor decreases. However, the decreased current from the field-effect transistor causes voltage drop across the resistor to decrease. With the opposite end of the resistor connected to the gate of the field-effect transistor, the relative increase in voltage causes an increased current flow through the field-effect transistor, compensating for the temperature fluctuation to stabilize the output voltage.
U.S. Pat. No. 4,794,247, issued on Dec. 27, 1988, to Stineman, Jr., discloses using an integrating amplifier with a feedback capacitor, to stabilize the bias signal from a photovoltaic detector, while reducing the noise effect.
U.S. Pat. No. 4,843,265, issued on Jun. 27, 1989, to Jiang, discloses a temperature compensating circuit that generates inverse variations in a field-effect transistor, achieved by charging a capacitor to a voltage and discharging the capacitor through a field-effect transistor in response to the fluctuations.
Also known to the field of art is the use of photovoltaic diodes to produce a current isolated from the current of the light source. Light sources capable of exciting current in photovoltaic diodes include light-emitting diodes. Prior art that demonstrates these uses include:
U.S. Pat. No. 5,805,062, issued on Sep. 8, 1998, to Pearlman, discloses an isolation amplifier that transmits data to a receiver via a current loop, where the isolated portion of the circuit is powered by a photovoltaic array illuminated by a light source, optionally an array of same frequencied light-emitting diodes.
A device is commercially available, referred to as an optically coupled floating power source, that is composed of one or more light-emitting diodes and one or more photovoltaic diodes, disposed within an opaque package in such a way that light from the light-emitting diodes impinge on the photovoltaic diodes, thereby generating a current in the photovoltaic diodes in response to the current supplied to the light-emitting diodes.
It would be an improvement to the field to create a bias voltage from a power source comprises of at least one light-emitting diode stimulating matched currents in at least two electrically isolated photovoltaic diodes, such that the circuit of one diode is used to provide a feedback voltage to an operational amplifier driving the light-emitting diode, thereby stabilizing the output voltage in both of the photovoltaic diode circuits.
It would be a further improvement to provide a distance between the bias source and detector due to the temperature of the detector. Such distance however typically requires shielding of the connection between the electronics and the detector, typically by coaxial cabling. The use of such shielding introduces capacitance which creates a pathway into the electronics for current noise resulting from the voltage noise in the bias generator. Low noise in the bias generator therefore becomes more critical under these circumstances.